const fp = require('lodash/fp');
const fs = require('fs')

class IO {
  static of(value) {
    return new IO(function () {
      return value;
    })
  }
  constructor(fn) {
    this._value = fn;
  }

  map(fn) {
    return new IO(fp.flowRight(fn, this._value));
  }

  join() {
    return this._value();
  }

  flatMap(fn) {
    return this.map(fn).join();
  }
}

let readFile = function (filename) {
  return new IO(function () {
    return fs.readFileSync(filename, 'utf-8')
  })
}

let print = function (x) {
  return new IO(function () {
    console.log('print', x);
    return x;
  })
}

let r = readFile('package.json')
  .map(fp.toUpper)// 返回值使用map
  .flatMap(print)// 返回函子使用flotMap
  .join()
console.log(r)